The present invention relates to forming fabrics for paper-making machines, this term being understood in its widest sense to include in particular machines for the manufacture of paper pulp, cellulose, Kraft paper, cardboard etc. It relates more particularly to endless multiply or multilayer forming fabrics having at least two piles or layers of transverse threads (i.e. threads running transversely to the direction of movement of the forming fabric in the paper-making machine) and one layer or ply of longitudinal threads (i.e. running in the direction of movement of the fabric during operation).
In a paper-making Fourdrinier machine the paper sheet is generally formed either on an endless forming fabric, or between two endless forming fabrics, or between an endless forming fabric and other members such as hollow fabric-covered cylinders or heated cylinders. The endless fabrics may be entirely of threads or yarns of synthetic or metal material or they may include, in combination, metal threads and mono- or multifilament synthetic threads. The multifilament synthetic threads are obtained by spinning out synthetic fibres or endless synthetic fibres; they may also be made by twisting such threads. The synthetic material is generally polyester or polyamide although other polymers or copolymers can be used. Before or during manufacture of the forming fabrics, the threads may be coated with a synthetic material.
The forming fabrics for paper-making machines must fulfil conditions which are to some extent contradictory.
The transverse rigidity of the forming fabric should be as high as possible, especially for fabrics of great length and width (respectively up to approx. 80 m and 10 m) which move at high speeds (e.g. 1100 m/min.). Insufficient transversal rigidity may result in the formation in the fabric of a fold which first deteriorates the sheet of paper formed on the fabric and ultimately ruins the fabric.
To increase rigidity, "double ply" fabrics have been used instead of single ply fabrics which have a single layer of longitudinal threads and a single layer of transverse threads. Such fabrics have two layers of transverse threads which both contribute to increase the transverse stiffness which is consequently much higher than in single ply fabrics.
For simplicity's sake, I shall call "upper layer" the layer of transverse threads located on the paper side when the fabric is on the paper-making machine. The lower layer of transverse threads will be the layer which, on the paper-making machine, is on the side of the fabric supporting members and dewatering elements (foils, suction boxes and the like).
It is well known that the fabric must impress on the material to be formed (typically a sheet of paper) a roughness or "mark" as faint as possible. Numerous attempts have been made to reduce the mark. Grinding the fabrics prior to use to make smoother the surface in contact with the paper has been proposed. But while the marks are less deep, they are wider. The number of threads per unit of surface has been increased to support the sheet of paper on a fabric having as many contact points as possible with the paper sheet but it results in a finer, therefore less resistant, fabric.
Up to now, it was generally felt that, in order to reduce the mark, it was advisable to increase the number of crossing points between longitudinal and transverse threads of the upper layer and to arrange that the outer surfaces of the undulations formed by all the threads towards the side of the sheet of paper are approximately tangential to a same plane. Such an assumption has led to use double layer fabrics wherein the longitudinal threads and the transverse threads of the upper layer are interwoven according to a "plain" weave at each crossing point, i.e. each longitudinal thread or yarn binds separately with one transversal thread or yarn of the upper layer at each crossing point.
Furthermore, the paper forming fabrics must have as high a resistance to abrasion as possible. In paper-making machines, the fabrics are supported on the inside by abrasive members, some of which support the fabric to keep it flat while others exert a suction on it to eliminate through the fabric a great part of the water in which the cellulose fibres and additives forming the paper pulp are in suspension.
The dewatering elements wear the fabric which must be replaced after a period of use. One of the principal concerns of manufacturers has been to increase the resistance to abrasion of the forming fabrics. For that purpose, attempts have been made to replace polyester yarns with polyamide yarns which are more resistant to abrasion. Unfortunately, the more abrasion resistant yarns are more flexible than the polyester yarns and it has been found that polyamide fabrics lack transverse stiffness and stretch too much under the tension necessary for use in paper-making machines.
It is a first object of the invention to provide an improved multilayer fabric for forming paper and the like which has a high transverse stiffness and at the same time does not impress an excessive mark on the paper.
It is another object to provide a multiply or multilayer forming fabric for a Fourdrinier machine which has a high resistance to wear by abrasion and consequently a long useful life.
An important step in conceiving the present invention was the determination of the surprising fact that the transverse threads mark the paper more than the longitudinal threads, even when the sheet of paper is supported by a fabric whose longitudinal and transverse threads have outer surfaces which are tangential to a same plane on the paper supporting side. That may be due to the anisotropy of the paper caused by the orientation given to the cellulose fibres during manufacture of the paper. That finding is used by providing a multilayer fabric in which the threads are so interwoven that the outer loops formed by the longitudinal threads contacting the paper on the paper supporting side of the fabric, cover from three to seven transverse threads of the upper layer, the first crossing points of said outer loops and transverse threads being distributed according to a repeating weaving pattern comprising at least five longitudinal threads. The weaving pattern on the paper side of the fabric is additionally selected so as to avoid full alignment of said outer loops on adjacent threads and diagonal effects. The outer loops formed by the longitudinal threads on the machine side of the fabric pass each time only under a single transverse thread of the lower layer. The outer loops are so arranged that there is no gap in the direction of the longitudinal threads between the outer loops of each longitudinal thread and the outer loops of the adjacent longitudinal threads.
Preferably, the "filling coefficient" for the longitudinal threads (i.e. the cumulative width of the longitudinal threads--supposed to be side by side--per unit width of the fabric) is between 1.05 and 2.0.
The fabric typically has two layers of transverse threads and a layer of longitudinal threads all of synthetic material, is flat woven and junctioned to make it endless. The upper loops formed by the longitudinal threads (warp threads) of synthetic material on the paper receiving side of the fabric cover from three to seven transverse threads (generally three or four transverse threads) of the upper layer. The first crossing points of the loops with the transverse threads may be distributed according to a satin weave of at least five threads. The lower loops formed by the longitudinal threads on the machine side of the fabric pass each time only under one transverse thread of the lower layer.
Advantageously, the transverse threads of the lower layer may have different characteristics from those of threads used for the other layer(s) of transverse threads and may have a higher resistance to abrasion. The transverse threads of at least one of the layers can have a diameter greater than that of the longitudinal threads, the ratio between the diameters being then advantageously equal to 1.05 at least and 2.5 at most, different threads may be used (for instance alternate polyester and polyamide threads).
The forming fabric is advantageously woven, or treated after weaving (e.g. subjected to a fixing heat treatment while maintained under longitudinal tension) under such conditions that, when the fabric is new, only transverse threads are in contact with the fabric supporting members of the machine.
The invention will be better understood from the following description of particular embodiments given by way of examples.